Dustin Johnson lines up for a putt on the 18th green during the second round of the HSBC Champions in Shanghai, China, Friday.

SHANGHAI — Dustin Johnson tied the course record with a 9-under 63, four shots better than anyone else Friday, and built a five-shot lead in the HSBC Champions. It was the kind of score that might make everyone else wonder which course he was playing.

Except that Graeme McDowell saw the whole thing.

Crouched behind the 10th green at Sheshan International, McDowell looked over at the powerful American and said, “I’ve probably seen 18 of the best drives I’ve seen all year in the last two days.” Moments later, after Johnson blasted another one down the middle, McDowell saw No. 19.

It was an impressive display, Johnson at his very best with the most important club in his bag. He ran off six birdies in his opening seven holes. He never hit more than 8-iron into a par 4. He twice came within 15 yards of driving the green on par 4s — once with a 3-iron.

Johnson became the sixth player with a 63 at Sheshan International, last achieved by Martin Kaymer in the final round in 2011. He was at 12-under 132, five shots clear of Rory McIlroy (72), Bubba Watson (69) and Boo Weekley (67).

McIlroy couldn’t keep up. No one could.

McIlroy, who started the second round of this World Golf Championship with a two-shot lead, made three birdies on the front nine to stretch his lead to four shots. At that point, Johnson was just getting warmed up. And it wasn’t long before McIlroy started to cool off. He drove into the same bunker he was in on Thursday and made another bogey, and he didn’t give himself any birdie chances over the last six holes.

On the par-5 14th, his wedge went just over the green and forced him to hit a delicate chip to save par. On the reachable par-4 16th, he drove into a tiny pot bunker left of the green and took two to get out, again scrambling for par. And on the par-5 18th, McIlroy hit his second shot into the water and made bogey.

“I just hit a couple of bad shots, and then I guess I let it affect me a little bit, and then I started to doubt myself sometimes,” McIlroy said. “I didn’t hit a lot of quality shots on the back nine.”

At least he’s still in the hunt for his first win of the year.

And if Johnson loses his stride, there are plenty of players behind him who are poised to make a move.

McDowell, Ernie Els, Sergio Garcia, Gonzalo Fernandez-Castano and Ian Poulter were among those at 6-under 138. Phil Mickelson, trying to recover from his quadruple bogey-bogey finish to the opening round, had a 68 and was seven shots behind.

“I’m making a lot of birdies,” Mickelson said. “But I’ve had too many mistakes. I’m going to continue to play aggressively out here, because I have to make birdies to catch the lead. And hopefully, I’ll eliminate a few mistakes here or there.”

The one positive from his finish Thursday? The last time Mickelson made a quadruple bogey was in the first round of the 2009 Tour Championship. He went on to win.

Even so, catching Johnson suddenly looks like a monumental task.

“It was kind of tough to focus when playing with Dustin Johnson hitting it 350 yards in the middle of the fairways,” McDowell said after his 69. “It was a little demoralizing at times. But I was happy that I hung in there. If he keeps playing the way he’s played the last two days, he’ll be tough to catch.

“What’s possible for an athlete like him who just rips it, that’s not what’s possible for the rest of us sometimes,” McDowell added. “It was probably one of the best driving displays I’ve seen this year, for sure. It was pretty immense. He’s got a short game to match, as well. When he’s in the mood and when he’s on form, he’s pretty prolific.”

Johnson wasn’t in much of a mood when he took double bogey on his fourth hole of the tournament and made the turn in 1-over. Starting with his eagle on the par-5 second hole of his opening round, he has played his last 26 holes in 13-under par.

His tee shot was so long on the 10th hole that it was through a cart path that crosses the fairway. His only mistake was forgetting that the green falls off to the right, which he didn’t realize until his shot tumbled down a steep slope. That led to his only bogey.

About the only thing that really tripped him up was a question from a Chinese reporter asking if he was aware he had so many female fans in China and if he brought his girlfriend with him. Indeed, Paulina Gretzky followed him around the course.

“It’s my fiancée,” Johnson said.

Weekley is no stranger to China, having played the World Cup at Mission Hills. He qualified for the HSBC Champions by winning Colonial, and he has played solidly for two days.

Johnson did not make the Presidents Cup, so he had a month off before going to Perth for a European Tour event. This is his first PGA Tour event of the 2013-14 season. He won the first event of the 2013 season, at wind-blown Kapalua in the 54-hole Tournament of Champions.

“It still hasn’t hit me that this is a new season,” he said. “I’d like to get off to a good start. I just want to play well the next two days.”